The invention generally relates to wireless time division duplex (TDD) communication systems using code division multiple access. In particular, the invention relates to sub-channels for the physical random access channel (PRACH) for such systems.
In code division multiple access (CDMA) communication systems using frequency division duplex (FDD), such as proposed for the third generation partnership project (3GPP), physical random access channels (PRACHs) are used for transmitting infrequent data packets and system control information from the user equipments (UEs) or users to the Node-B.
In a 3GPP FDD/CDMA system, the PRACH is divided into ten (10) millisecond radio frames 221 to 228 (22) having fifteen (15) timeslots 24, as shown in FIG. 1. The radio frames 22 are sequentially numbered, such as numbered from 0 to 255, as a system frame number. The system frame numbers are sequentially repeated. The random access transmission starts at the beginning of a number of well-defined time intervals, denoted access slots 26. The random access transmissions 281 to 285 (28) from the users are begun in a particular access slot 26 and continue for one or multiple slots 26. These transmissions are sent using a randomly selected signature associated with an access service class (ASC) assigned by a radio resource controller of the network to the user.
The PRACH is used for infrequent data packets and system control information and the network uses sub-channels of the PRACH for further separation of UEs and access service classes. In the 3GPP FDD/CDMA system, each sub-channel is associated with a subset of the total uplink access slots 26, described as follows.
Two sequential radio frames 22 are combined into one access frame 20. The access frame is divided into 15 access slots 26. Each access slot 26 has a duration of two radio frame timeslots 24 as shown in FIG. 1. The duration of a radio frame 22 is shown in FIG. 1 by the dual headed arrows. The sub-channels are assigned to the access slots 26 by sequentially numbering the slots from 0 to 11, as shown in FIG. 1. After sub-channel 11 is assigned, the next access slot 26 is numbered 0 and the numbering is repeated. The access slot 26 to sub-channel numbering is repeated every 8 radio frames or 80 milliseconds (ms). This repetition can be viewed as a modulo (mod) 8 counting of the radio frame numbers.
In 3GPP FDD/CDMA, multiple PRACHs are used. Each PRACH is uniquely associated with a random access channel (RACH) transport channel and is also associated with a unique combination of preamble scrambling code, available preamble signatures and available sub-channels.
FIG. 2 is one example of an illustration of such an association. RACH 0300 is paired with PRACH 0320 through a coding block 310. The data received over PRACH 0320 is recovered using the preamble scrambling code 0340 and the appropriate preamble signature 38 that the data was sent.
PRACH 0320 is uniquely associated with preamble scrambling code 0340 and has three access service classes (ASCs), ASC0400, ASC1401 and ASC2402. Although the number of ASCs shown in this example are three, the maximum number of ASCs is eight (8). Each ASC 40 has a number of available sub-channels, available preamble signatures and a persistence factor. The persistence factor represents the persistence in retransmitting the preamble signature after a failed access attempt. In 3GPP FDD/CDMA, the maximum available sub-channels 36 is 12 and the maximum available preamble signatures 38 is 16.
RACH 1301 is paired with PRACH 1321. PRACH 1321 is uniquely associated with preamble scrambling code 1341 and its sub-channels 36 and preamble signatures 38 are partitioned into four ASCs 40, ASC0403, ASC1404, ASC2405 and ASC3406. RACH 2302 is paired with PRACH 2322. PRACH 2322 uses preamble scrambling code 2342, which is also used by PRACH 3323. Three ASCs 40 are available for PRACH 2322, ASC0407, ASC1408 and ASC2409. Because PRACH 2 and PRACH 3 share the preamble scrambling code, a group of partitioned off available sub-channels/available preamble signature combinations are not used for PRACH 2322. The partitioned off area is used by PRACH 3323.
RACH 3303 is paired with PRACH 3323. PRACH 3323 also uses preamble scrambling code 2342 and uses ASC04010 and ASC14011. ASC04010 and ASC14011 contain the available sub-channel/signature set not used by PRACH 2323.
Since each PRACH ASC 40 is uniquely associated with a preamble scrambling code 34 and available preamble signatures set and sub-channels, the Node-B can determine which PRACH 32 and ASC 40 is associated with received PRACH data. As a result, the received PRACH data is sent to the appropriate RACH transport channel. Although each PRACH 32 is illustrated in this example by having the ASCs 40 partitioned by available preamble signatures, the partitions may also be by sub-channel 36.
Another communication system proposed to use PRACHs is a CDMA system using time division duplex (TDD), such as the proposed 3GPP TDD/CDMA system. In TDD, radio frames are divided into timeslots used for transferring user data. Each timeslot is used to transfer only uplink or downlink data. By contrast, an FDD/CDMA system divides the uplink and downlink by frequency spectrum. Although the air interface, physical layer, between FDD and TDD systems are quite different, it is desirable to have similarities between the two systems to reduce the complexity at the network layers, such as layer 2 and 3.
Accordingly, it is desirable to have sub-channels for the RACH for TDD.
Sub-channels are defined for a physical random access channel of a wireless time division duplex communication system using code division multiple access. A series of radio frames have a sequence of timeslots. For a particular timeslot number of the sequence, each sub-channel of the particular timeslot number is uniquely defined by one radio frame of the series.